feat(test): add legacy YAML migration command

[xiong35]

Summary

Add a migration command for converting legacy Midscene Web YAML suites into the new @midscene/test project format.

midscene-test migrate <legacy-yaml-or-directory> \
  --output-dir <new-test-project>

What changed

• Add the migrate CLI command.
• Generate:
  • midscene.config.ts
  • cases/*.yaml
  • MIGRATION.md
• Convert supported legacy actions:
  • ai / aiAction / aiAct → aiAct
  • aiAssert → aiAssert
  • sleep → wait
  • recordToReport / logScreenshot → recordToReport
• Migrate Web target settings, concurrency, retry, bail, Agent options, and environment references.
• Handle generated Project-name and filename collisions.
• Preserve legacy step environment-variable fallback behavior.
• Add atomic output generation:
  • legacy source files are never modified
  • existing output directories are never overwritten
  • no partial output is retained when migration fails
• Report field-level blocking errors for behavior that cannot be migrated losslessly.
• Emit migration warnings both in the CLI and generated MIGRATION.md.
• Add migration documentation and CLI/unit test coverage.

Compatibility and limitations

The automatic path currently targets legacy Web YAML.

Unsupported or semantically different behavior fails closed and requires manual migration, including:

• custom legacy actions such as javascript, query/wait/input actions, or Gherkin steps
• setup / serve
• shared browser context
• cookie and download handling
• Bridge/CDP configuration
• non-Web platforms
• typed environment references whose YAML parsing semantics cannot be preserved
• keepWindow and unsafe continueOnError combinations

These cases should be implemented with defineProjectSetup, Playwright, or custom Test Runner Nodes after generating the supported portion.

describe-nodes remains an optional documentation and configuration-inspection command; it does not affect test execution.

Validation

• pnpm run lint
• pnpm exec nx test @midscene/test
  • 21 test files passed
  • 191 tests passed
• pnpm --filter @midscene/test build

Additional demo validation covered 45 migration scenarios:

• 10 expected successful migrations
• 35 expected atomic rejections
• 0 unexpected results

Real Chromium validation also passed for environment-variable compatibility and manually migrated lifecycle/custom-node scenarios.